The present invention relates to porous hollow polypropylene filaments and a method of making the same.
U.S. Pat. No. 3,558,764 discloses a method for producing a porous film by stretching a film comprising polypropylene and other crystalline high polymers to produce pores in the film. According to this method, the porous polypropylene film obtained has pores of 1000-2000 A in average diameter which communicate with each other. Therefore, this film can be effectively used as a gas separating film, ultrafilter, reverse osmosis membrane support, etc.
However, when the porous polypropylene film obtained by said method is used, e.g., as the ultrafilter (2400 W film manufactured by Celanese Corp.), the permeation rate of water is about 0.02-0.03 g/cm.sup.2.min.atm and thus such film is lower than the conventional cellulose acetate ultrafilter in flux (gallon of water which permeates the film area of 1 feet square per 1 day) and there is the problem in the cost in treating water in an industrial scale. However, when such film of the low flux is made into hollow filaments, film area per volume of apparatus can be markedly increased and amount of water which can be treated per unit volume of the apparatus is increased. Thus, it becomes economically possible to carry out the treatment of water in the industrial scale.